This invention relates to lithographic printing plate immersion type processors and more particularly to a lithographic printing plate processor having an improved developer exit tank isolated from the developing tank.
In the lithographic printing process, a lithographic plate comprising an aluminum sheet is initially coated with a photosensitive layer usually containing, among other things, one or more resins. A negative is applied over the photosensitive layer, and the two are exposed to high intensity light. Alternatively, the coated plate may be exposed to imaging radiation directly using an imaging laser whose beam is scanned and modulated to form an image on the plate. The exposed plate is processed to develop it. In development, the image areas of the photosensitive layer are retained on the plate surface and the non-image areas are removed. Although some manual developing is still done, the large scale commercial use of lithographic plates has made automatic processing of necessity. Numerous apparatus have been proposed to perform such processes.
In known apparatuses of this type, the processing liquid used for developing printing plates is contained in a developing tank into which a plate to be developed is immersed during the development step. When spraying developer onto the plate surface is part of the process, processing liquid is typically pumped from the developing tank to the sprays by means of a circulating pump and a filter. U.S. Pat. No. 3,589,261 issued Jun. 29, 1971 to Krikelis discloses a typical immersion processor useful for developing a lithographic printing plate.
In an effort to preserve developer and comply with environmental regulations most processors today employ developer recirculation and filtration of the developer. In a typical such system, at least one filter is arranged in the flow path between the tank and the printing plate developing tank, to protect the printing plate being developed from dirt particles and loosened particles of the layer which are washed away with the processing liquid. Normally, the filter is positioned on the delivery side of a circulating pump. U.S. Pat. No. 4,239,368 issued Dec. 16, 1980 to Krause et al., discloses such a recirculation/filtration system for use with printing plate processors.
A number of processors employ a developer exit tank immediately following the developing tank in which the developed printing plate is subjected to a rinse with clean developer or water. FIG. 1 shows a schematic elevation of the portion pertinent to the present invention of an immersion type developing apparatus commercially available under the name Mercury of the Americas made by Heights Design Production Limited of Halifax, West Yorkshire, UK.
As shown in FIG. 1 the developer apparatus 10 comprises a developer tank 12 which contains developer solution 14 into which a lithographic printing plate 16 is immersed. A number of drive and guide rollers 18 drive and guide the printing plate through the developer tank 12. At the exit of the developing tank 12 there is a developer exit tank 20. A pair of squeegee rollers 22 serve to remove remaining developer from the plate surface, before the plate enters the developer exit tank 20.
The exit tank 20 is in communication with the developing tank 12 through a port 26 designed to return the developer to the developing tank 12. The port 26 may be closed and the developer exit tank 20 may be disconnected from the main tank and may be connected through a valve 23 to a drain 24 and therefrom to a desired container (not shown) for emptying the exit tank. This latter feature is primarily intended for use when instead of using developer solution for rinsing the plate exiting the developer tank, the plate is subjected to a water rinse; in such case the drain 24 is intended to be connected to a water disposal drain such as a sewer line.
The processor further includes a pump 34 and a filtration system 36. A developer recirculation loop is formed by connecting the developer tank 12 to pump 34 through pipe 33; the pump output is connected to filtration system 36 through pipe 35; pipe 37 brings filtered developer back to the developer exit tank 20 where developer is applied to the developed plate through a spray system 38.
Such arrangement is unsatisfactory. Developer in the developing tank contains a large amount of loosened particles of the layer that is being processed. Using the same developer both in the immersion section of apparatus and in the exit section results in the transfer of such particles back onto the developed surface. Proposed solutions to date appear to require the use of rather sophisticated, and therefore expensive, filtration methods to generate the clean developer required for this stage of the plate processing, as exemplified by the filtering system shown in aforementioned Krause et al patent if one is to avoid the equal expensive and usually time consuming process of frequently changing filters in less sophisticated systems.
The typical processor of the prior art will also comprise additional processing stations 28 and 30 which may provide additional rinsing, gum application etc. Such additional stations are not relevant to the subject matter of the present invention.
In an effort to provide a practical solution to the aforementioned problem there is provided a lithographic printing plate processor of the type comprising an immersion developing tank followed by a developer exit tank in which a printing plate driven through the immersion tank is subjected to a spray of clean developer wherein the exit tank is isolated from the developing tank and there is associated with the exit tank developer spray system a clean developer recirculating loop comprising a holding tank, a pump and a filter, said clean developer recirculating loop being isolated, separate and distinct from said developing tank.
Similarly in accordance with this invention there is provided a method for simplifying the filtering requirements of the developer circuit in processors comprising an immersion developing section followed by a clean developer exit tank the method comprising isolating the developer used in the exit tank from the developer used in the developing tank by providing separate and distinct developer circulation loops for the developer used in the developing tank and the clean developer used in the exit tank.